The present invention pertains generally to the field of implantable biosensors and, in particular, to methods and apparatus for locating a biosensor at an implantation site in the body for monitoring physiological conditions in a patient.
An aneurysm is an abnormal ballooning of the wall of an artery that results from the weakening of the artery due to injury, infection, or other conditions, such as a congenital defect in the arterial connective tissue. Common forms of such an aneurysm include an abdominal aortic aneurysm, an iliac aneurysm, a bifurcated aneurysm of the abdominal aorta and the iliac, and a thoracic aortic aneurysm.
The aorta, which is the main arterial link in the circulatory system, begins at the left ventricle of the heart, forms an arch above the heart, and passes behind the heart, continuing downward through the thorax and the abdomen. Along this path, the abdominal aorta branches into two vessels, called the renal arteries, that supply blood to the kidneys. Below the level of the renal arteries, the abdominal aorta extends approximately to the level of the fourth lumbar vertebra, where it branches into the iliac arteries. The iliac arteries, in turn, supply blood to the lower extremities and the perineal region.
Abdominal aortic aneurysms can occur in the portion of the abdominal aorta between the renal and the iliac arteries. This condition, which is most often seen in elderly men, often leads to serious complications, including rupture of the aneurysmal sac. A ruptured aneurysm occurs in approximately 3.6 out of 10,000 people and is considered a medical emergency, since the resultant rapid hemorrhaging is frequently fatal.
There are generally two methods for treating abdominal aortic aneurysms: (1) surgical repair of the aneurysm, and (2) endoluminal stent graft implantation. Surgical repair of the aneurysm involves the implantation of a tubular prosthetic vascular graft, traditionally made of fluoropolymers, such as polytetrafluoroethylene (PTFE) or polyester (Dacron), into the aorta. These prosthetic vascular grafts traditionally have been implanted by open surgical techniques, whereby a diseased or damaged segment of the blood vessel is surgically cut along its longitudinal axis and the tubular bioprosthetic graft is then inserted coaxial to the original artery and anastomosed within the host blood vessel as an internal replacement for the diseased segment. Then the longitudinal cut in the artery is sutured. Alternatively, prosthetic vascular grafts have been used as bypass grafts wherein opposite ends of the graft are sutured to the host blood vessel in order to form a conduit around the diseased, injured, or occluded segment of the host vessel.
These surgical approaches suffer from similar disadvantages, namely, the extensive recovery period associated with major abdominal surgery, the difficulties in suturing the graft to the aorta, the unsuitability of surgery for many at-risk patients, and the high mortality and morbidity rates associated with surgical intervention of this magnitude.
The second approach to treating an abdominal aortic aneurysm, endolumenal stent graft implantation, overcomes many of these disadvantages. An endoluminal stent graft normally consists of a vascular graft that is supported by a metallic stent skeleton over a portion of the length of the graft. By introducing and deploying the stent graft through the lumen of the blood vessel, a surgeon may then repair the damaged aortic segment using only percutaneous or minimal incisions in the patient. This technique initially involves translumenal delivery of the graft in a compacted low profile configuration by way of a catheter or some other transluminally advancable delivery apparatus. The stent is then radially expanded, thereby anchoring the graft to the surrounding blood vessel wall and sealing off the aneurysm from the rest of the circulatory system. As a result, the pressure within the isolated aneurysmal sac and the endotension of the artery are both reduced.
It is generally agreed that such endoluminal stent grafts work best in patients with small- to medium-sized abdominal aortic aneurysms, or in patients with large abdominal aortic aneurysms who are characterized as high risk candidates for open surgical abdominal aortic aneurysm repair. In addition to treating vascular aneurysms, an endovascular stent graft may also be used to treat occlusive vascular disease.
In some instances, the stented graft is constructed in such a manner that the tubular graft material forms a complete barrier between the stent and the blood, which is flowing through the blood vessel. In this way, the tubular graft material serves as a smooth, biologically compatible inner lining for the stent. Graft material known in the prior art includes woven or knitted fabrics, such as polyester fiber, or a porous form of PTFE known as ePTFE.
The major complication involved in the endolumenal stent graft implantation is the formation of an endoleak. An endoleak is defined as blood leakage into the aneurysmal sac causing the sac to fill with blood and increasing the endotension. Endotension is defined by the internal pressure within the aneurysm, the aneurysm diameter and wall thickness. In particular, endotension is a physical parameter that indicates the chances of aneurysm rupture. The implantation of a stent graft prevents blood from filling the aneurysmal sac, resulting in a depressurization of the sac with minimal influence on the aneurysm wall thickness. The diameter of the aneurysm might change with pressure reduction, but the direct parameter that varies is the pressure.
Endoleaks can be divided into four categories: Type I, which results from leakage due to insufficient sealing of the graft against the aortic wall; type II, which results from blood flow to the aneurysmal sac through bypass arteries; type III, which arises from mechanical failure of the graft system; and type IV, which arises from leakage through the graft fabric due to the porosity of the material.
Because of the high risk of aneurysmal rupture, the early detection of endoleaks resulting in endotension is crucial. With early detection, the pressure within the aneurysmal sac may be reduced through endovascular treatment (balloon inflation or additional stent graft implantation for improve sealing) or a surgical intervention. Currently, the standard method for the detection of endoleaks is through contrast-enhanced computerized tomography (CT), which relies on the x-ray imaging of the abdominal region after injection of a contrast media in order to improve the detection of blood and vascular tissue. If an endoleak is present, then the aneurysmal sac will fill with contrast media and the endoleak will then be identified in the resultant CT scan.
Although CT scans are considered a reliable method for detecting endoleaks, they suffer from several disadvantages. First, CT scans require an experienced operator and an expensive apparatus, placing significant financial constraints on its frequency of use. Second, the CT scan procedure exposes the patient to x-ray radiation and thus cannot be used as frequently as desired. Third, CT scans can only provide an estimate of the pressure within the aneurysm indirectly by detecting leakage into the aneurysmal sac, and are unable to detect small leaks that may cause slow, but potentially dangerous, pressurization within the aneurysm.
In addition to CT scans, ultrasound imaging methods have also been used to detect endoleaks. Ultrasound-based methodologies posses several advantages over CT, including a simpler apparatus and the absence of ionizing radiation. Consequently, such imaging can be performed more often and at a lower cost than CT scans. However, ultrasound-based imaging is operator dependent and less reliable than CT scans.
Thus, there exists a need for more accurate and reliable methods and apparatus for detecting endoleaks. More particularly, there exists a need for directly monitoring the internal pressure within an aneurysmal sac in order to determine the presence or absence of an endoleak or endotension at a higher frequency.
In accordance with one aspect of the invention, a device for delivering a biosensor to an implantation site in a body is provided, comprising an elongate catheter comprising a recess configured to carry the biosensor while the catheter is guided to the implantation site. In one preferred embodiment, the recess comprises a longitudinal indentation etched or otherwise formed in a side of the catheter. In another preferred embodiment, the recess comprises a circumferential indentation formed in a side of the catheter. In still another preferred embodiment, the recess comprises a cavity formed in the catheter. In yet another preferred embodiment, the recess comprises a cavity formed in a distal tip of the catheter.
In preferred embodiments, the implantation device may also include a retaining element configured to retain the biosensor in the recess. In one preferred embodiment, the retaining element comprises a thin membrane at least partially covering the recess. In another preferred embodiment, the retaining element comprises a retractable sheath extending out of a distal opening of the catheter. In yet another preferred embodiment, the retaining element comprises a retractable filament inserted through a distal opening of the catheter. A clamping mechanism may also be provided, which is adapted to secure the retaining element against the catheter. By way of one non-limiting example, the clamping mechanism may comprise a sleeve circumferentially attached to the catheter.
In preferred embodiments, the implantation device may also include an actuator disposed in, or adjacent to, the recess, the actuator configured to eject the biosensor from the recess. By way of non limiting examples, the actuator may comprise a piston or a spring. In one preferred embodiment, the actuator comprises a protrusion located in, or adjacent to, the recess, which is positioned to displace the biosensor from the recess. The implantation device may also be provided with a handle assembly associated with the actuator, the actuator being controllable by manipulation of the handle assembly.
In accordance with another aspect of the invention, a method for using an implantation device to deliver a biosensor to an implantation site in a body is provided, the implantation device comprising an elongate catheter having a distally located recess configured to carry the biosensor, the method including the steps of introducing the catheter into the body with the biosensor disposed in the recess, until the recess is positioned at the implantation site, and then displacing the biosensor from the recess into the implantation site.
In preferred implementations of the method, the implantation device includes an actuator disposed in, or adjacent to, the recess, wherein the ejecting step is performed with the actuator. In one preferred implementation of the method, the implantation device further includes a thin membrane at least partially covering the recess, wherein the actuator, during the ejecting step, causes the biosensor to be pushed through the thin membrane and into the implantation site. In another preferred implementation of the method, the implantation device further includes a retractable retaining element configured to retain the biosensor in the recess during the introducing step. In accordance with yet another aspect of the invention, the catheter of the implantation device is guided to the implantation site in conjunction with the delivery of a stent graft.
Notably, the implantation site may be an abdominal aortic aneurysm, in the iliac of a bifurcated abdominal aortic aneurysm, or a thoracic aortic aneurysm, or some combination thereof. As will be appreciated by those skilled in the art, however, the inventive aspects disclosed and described may be applied to the placement of a biosensor in any implantation site in a body, and are not restricted to abdominal or aneurysmal implantation sites.
In accordance with still another aspect of the invention, a method using an implantation device for delivering a biosensor to an implantation site in a body is provided, the implantation device comprising an elongate catheter having a distally located recessed area configured to house the biosensor, and a retractable retaining element configured to retain the biosensor in the recessed area, the method including introducing the catheter into the body, with the biosensor retained in the recessed area by the retaining element, until the recessed area is positioned at the implantation site, and retracting the retaining element so that the biosensor may move freely from the recessed area into the implantation site.
In accordance with yet another aspect of the invention, a method using an implantation device for delivering a biosensor to an implantation site in a body is provided, the implantation device comprising an elongate catheter having a distally located recess configured to at least partially house the biosensor, a retractable cover member configured to retain the biosensor within the recess, and an actuator configured to displace the biosensor from the recess, the method including introducing the catheter into the body with the biosensor retained within the recess by the cover member, until the recess is positioned at the implantation site, retracting the cover member to allow passage of the biosensor from the recess, and ejecting the biosensor from the recess into the implantation site with the actuator.
As will be apparent to those skilled in the art, other and further aspects of the present invention will appear hereinafter.